1. Field of the Invention
The present invention relates generally to lens blanks for forming ophthalmic elements and the lamination of thermoplastic molded articles in the formation of ophthalmic elements. The invention is particularly designed to provide a finished multifocal or progressive lens with anti-reflective or abrasion resistant coatings at a commercial shop in less than twenty minutes.
2. Background of the Art
The present invention finds utility in a range of materials to be joined by adhesive securement, and it is applied most advantageously to molded thermoplastic articles such as lenses, especially ophthalmic lenses, and optical disks.
Lenses are used for a wide variety of purposes. For example, microscopes, telescopes, magnifying glasses and other optical instruments, as well as ophthalmic spectacles, employ lenses. The following discussion focuses on the most preferred embodiment of the present invention, ophthalmic lenses.
Vision-corrective, prescription (Rx) spectacle lenses or ophthalmic lenses increasingly employ plastic lens materials instead of the more traditional glass. In fact, in the United States, the demand for plastic lenses is multiples of that for glass lenses. This is because:
1. plastic is lighter than glass, which is particularly advantageous where stronger prescriptions and thicker lenses must be used; PA1 2. durable abrasion-resistant coatings have become available for plastic; PA1 3. plastic can be provided in a wide range of fashionable colors and gradient-density tints; and PA1 4. production techniques have improved so that plastic lenses can be manufactured at higher rates, with more automation, and lower costs. PA1 a. holding a front face of the front lens in a mounting aperture positioned relative to a laminating axis; PA1 b. mounting a rear face of the rear lens in a predetermined position on a holder that is movable along said laminating axis and holds the rear lens independently of the front lens; PA1 c. rotatably orienting the holder relative to said axis to bring the rear lens into a desired angular relationship with the front lens while the rear lens is separated from the front lens; PA1 d. placing a bonding material on a rear face of the front lens and moving the holder to bring a front face of the rear lens into engagement with the bonding material and to press the rear lens against the front lens in the direction of the laminating axis to force the bonding material to spread throughout a space between the two lenses; and PA1 e. curing the bonding material by directing curing radiation through the front lens while pressing and holding the rear lens against the front lens. PA1 a) roughness of a portion of a surface of one of the lens blanks, which portion of a surface is outside of a significant optical area of one of the lens blanks, and which roughness increases the static coefficient of friction between the two lens blanks; PA1 b) at least two protrusions extending from a concave surface of one of the lenses, the other of the lenses nesting within the concave surface, the at least two protrusions extending high enough from the concave surface to make contact with an edge of the other lens nesting within the concave surface; and PA1 c) protrusions extending away from a convex side of one of the lens blanks and over an edge of a lens blank nested against the convex side of one of said lens blanks.
The relatively lighter weight and corresponding improved wearer comfort of plastic lenses are the most important considerations by the consumer. Since a nominal lens thickness (typically 2.0-2.2 mm) is the same for glass and plastic, plastic lenses' lighter weight is a direct result of plastic's lower density compared to that of glass. This factor holds true for all equivalent prescriptions in glass and plastics, but becomes particularly important when higher-powered corrections are required or when larger spectacle frames are chosen, requiring larger lens diameters and requiring greater thickness at the exterior (or interior) portion of the lens to continue the prescription curvature of the correction.
One method by which plastic prescription lenses are currently manufactured is by individually casting polycarbonate resins, or casting and curing allylic (or other ethylenically unsaturated) resins. Polycarbonate thermoplastic lenses have started to replace both cast-thermoset plastic and traditional glass lenses because of their lower density and higher refractive index. Polycarbonate lenses of the same nominal thickness provide even lighter weight than the cast-allylic plastics, and are of a much lower weight than glass. Additionally benefits result from this resin class because polycarbonate tends to have far greater impact strength and breakage resistance than any clear polymeric materials presently used for lenses, and even thinner lenses (in the range of 1.5-2.0 mm) are presently available, with the potential for even higher wearer preference.
Another method by which ophthalmic lenses, and especially ophthalmic lenses having segmented (bifocal or trifocal) prescriptions or continuous prescriptions (with a smooth gradation between various prescriptions) can be manufactured is by laminating sets of at least one interior lens blank and at least one exterior lens blank, with each of the interior and exterior lens blanks providing a specific contribution to the final prescription. Composite eyeglass lenses have been formed with this principle by bonding together front and rear lenses, as suggested in U.S. Pat. No. 2,618,200. A device and method for accomplishing this process has been suggested in U.S. Pat. No. 4,927,480. Generally, the bonding process involves placing a curable adhesive on the concave interface surface of the front lens; pressing the convex interface surface of the rear lens against the adhesive in the front lens to spread the adhesive throughout the space between the two lenses; and curing the adhesive to bond the lenses together, forming a composite lens which is then trimmed to fit within an eyeglass frame.
Even after individual lens blanks of good optical properties have been manufactured, it is equally important to form them into ophthalmic lenses for use by the customer. Segmented and progressive ophthalmic lenses must also be capable of construction from these lens elements. For example, U.S. Pat. Nos. 4,883,548; 4,867,553; and 4,645,317 show the formation of laminated ophthalmic lenses from at least two separate lens elements which are selected from a reserve and then associated to match a particular description. The at least two lens elements (one front and one rear lens) are adhesively secured together, with a photosetting resin and photinitiator suggested for the process (e.g., U.S. Pat. Nos. 4,883,548 and 4,867,553).
Especially when the desired composite lens includes a cylindrical component that must be properly oriented to correct for astigmatism and a bifocal or progressive focal region that must be properly positioned for reading purposes, the existing methods and equipment have fallen short of the desired optical accuracy. Existing laminating equipment, for example, does not readily accommodate eccentric positioning and bonding of the front and rear lenses, which can be necessary in some cases. Also, existing methods and equipment have been inconvenient in operation and have put the desirable accuracies beyond practical reach for some composite eyeglass lenses. U.S. Pat. No. 5,433,810 describes lamination or bonding together of front and rear lenses to form a composite eyeglass lens to address these perceived problems. It is asserted in U.S. Pat. No. 5,433,810 that a described new and better way of mounting, aligning, and bonding together composite eyeglass lenses improved on the accuracy attainable. That composite lens laminating system includes a front lens platform on a movable stage and a rear lens holder that is rotationally adjustable on a laminating axis and is movable along the laminating axis to press the rear lens against the front lens. Accurate positioning of each lens is assured by an X, Y adjustment of the stage holding the front lens platform, by an arrangement of locating pins and pressure feet that register and hold the rear lens on its holder, and rotational adjustment of the rear lens holder on the laminating axis. A simple and effective arrangement assures that the two lenses are pressed together with uniform pressure that spreads the adhesive evenly between them, and this process can be observed through the rear lens while the pressure is being applied. Then the bonding adhesive is cured while the two lenses are pressed together so that the accuracy of their positioning and the lack of any interruption in the adhesive layer are preserved during the curing process. Several specific procedures and structures contribute to achieving these effects; and the result is described as fast, effective, and more accurate than previous systems. That method of laminating front and rear lenses to form a composite eyeglass lens comprises:
Polycarbonate's potential advantages over cast allylics were virtually offset by comparatively poorer abrasion resistance performance and poorer tintability, as well as restricted product line ranges and high manufacturing costs associated with low-volume production. Improved abrasion resistant coatings have overcome these limitations. Readily tintable coatings possessing good abrasion resistance have now become commercially available for polycarbonate lenses. Therefore the last major remaining drawback to the use of polycarbonate is associated with lens availability, breadth of product line, manufacturing costs, and order turnaround time.
The advantages and disadvantages associated with the use of polycarbonate are particularly pertinent and applicable to finished single-vision or multiple-vision lenses, which are supplied with one or both final-front-convex and back-concave optically finished surfaces, and optionally with a factory-applied tintable abrasion resistant coatings (e.g., often referred to in the art as "hardcoatings") on one or both surfaces. To convert such finished single-vision lenses (which constitute nearly half of all U.S. prescriptions filled) requires merely edging the excess lens away to fit a frame, and optionally tinting the lens to desired color with conventional dye baths, if the lens is not already tinted or photochromic.
Polycarbonate finished single-vision lens manufacturing has certain drawbacks which prevent their attaining lowest manufacturing costs and improved availability. A finished single-vision lens is optically defined by two measures of its light-bending power: spherical power (magnification) and cylindrical power (astigmatism correction), with units of power being read in diopters and various (e.g., 0.10, 0.25, etc.) units thereof. A product availability matrix which provides for sphere power ranging from at least about +4 to -6 diopters (with a broader range, e.g., from +6 to -8 easily possible) and cylinder power from 0 to +2 diopters (with a broader range to +3 diopters possible), constituting 273 or more (e.g., to 500) stock keeping units is desired. Within this matrix, there is a unit-volume frequency distribution curve which has at its approximate center a zero-power lens and which generally shows reduced frequency as sphere or cylinder power increases. To satisfy most incoming prescriptions on a statistical basis, a large matrix of stock keeping units must be maintained and inventoried for quick order turnaround if a particular manufacturer or lens type is to become popularly accepted in the market.
In addition to maintaining this wide range of product line, the lens manufacturer must necessarily produce high volumes of such thermoplastic-molded, hardcoated lenses.
Modern molding processes permit prescription lens molding at high yields, with minimal material scrap, without secondary operations of trimming, and with high levels of automation. Additionally, given the number of stock keeping units, each of which has a different statistical frequency distribution, it has become important to be able to run high volumes of differing lenses of differing powers (within some reasonable range) in a short period of time with minimum down time on the molding equipment, without sacrificing productivity, quality or yields. A four-cavity moldset, for example, quadruples the productivity associated with a particular molding machine without proportionately increasing its capital cost (i.e., increasing the capacity from one lens to four lenses may be less than 50% higher). For example, two of the cavities could be used to mold the most popular sphere and cylinder power combination and the remaining cavities could each handle a less popular lens, with more frequent changeovers of the latter cavities.
The lens blanks can be made by many different types of molding processes, including, but not limited to injection molding, stamping, coin pressing, and the like. For example, U.S. Pat. No. 4,828,769 discloses a method for injection molding articles, especially ophthalmic lens blanks, which method comprises forming a closed mold cavity for receiving plasticized resin without introducing significant back pressure therein, injecting into the closed mold cavity a mass of plasticized resin slightly larger than the mass of the article to be formed, applying a main clamp force of the injection molding equipment to reduce the volume of the closed mold cavity, thereby redistributing the resin contained within the cavity, and maintaining the applied main clamp force, thereby compressing the resin at least until the resin within the closed mold cavity solidifies. This one-step method is described as at least addressing some of the problems encountered in prior art injection molding processes when they had been attempted for use with ophthalmic lenses.
However, after the lens blanks are manufactured, it then becomes necessary to join at least the interior and exterior lens blanks to form the composite lens which can be a labor intensive and cost intensive step in the manufacturing process. U.S. Pat. No. 5,433,810 describes a process for the lamination of lens blanks into composite eyeglass lenses. A front face of a front lens is positioned in a mounting aperture relative to a laminating axis. A rear lens is mounted in a predetermined position on a holder that is movable along the laminating axis, with the holder supporting the rear lens (preferably independent of the front lens). The holder is rotatably oriented relative to the axis to bring the rear lens into a desired relationship with the front lens while the rear lens is separated from the front lens. A bonding material (e.g., UV curable adhesive) is placed on the rear surface of the front lens and the holder is moved to place the rear lens into engagement with the front lens and to force the bonding material to spread between the front and rear lens. The bonding agent is then hardened, as by exposing the bonding agent to curing radiation through the front lens.
U.S. Pat. No. 4,969,729 describes the lamination of lens blanks to form a composite lens by placing an adhesive between a front and rear lens then floating the lenses on a heated fluid to cure the adhesive. Indexing holes 17 are provided around the periphery of the lens blanks to assist in the proper relative optical positioning of the lenses with respect to each other. The lenses are rotated about an optical axis and the indexing holes are apparently used in assisting in the alignment of the lenses. A similar lamination process for adhering opposed etched surfaces of lens blanks without requiring the heating or floating in a liquid is described in U.S. Pat. No. 4,892,903, and indexing holes 17 are also described therein.
U.S. Pat. No. 5,399,227 describes a composite eyeglass lens laminating holder and a process used to laminate lens blanks to form a composite lens. The front and rear lenses of a composite ophthalmic lens are laminated together by holding the front lens in an X, Y adjustable stage on a laminating axis and holding a rear lens in a predetermined position relative to the laminating axis. The two lenses are moved together on the laminating axis to spread the adhesive. The rear lens holder is rotatable around a laminating axis and has an adjustable center foot adhesively tacked to the rear lens and an array of surrounding pressure feet for pressing the rear lens uniformly against the front lens. The adhesive between the two lenses is then cured by exposure to UV radiation through the front lens. It is preferred that the front lens has a tab or projection 18 which extends outward from its otherwise circular periphery, the tab to be received with a recess 19 to assure that the front lens has its bifocal region properly oriented on the platform supporting the lens.
It is of course desirable to provide alternative and more effective methods of laminating and curing ophthalmic lenses. One problem that has not been fully addressed by these references is the need to keep the front and rear lenses in center alignment during lamination. This type of centering would be especially critical where a centrifugal or rotational movement might be used to spread the adhesive between the lenses. Such rotational or spinning actions can readily shift the lenses out of their common ophthalmic or laminating axes, or out of ophthalmic alignment with respect to the desired positioning of the lens blank elements when laminated.